The Cookie Prefixes spec disallows the use of the `domain` attribute in cookies
if the `__Host-` prefix is used
(https://tools.ietf.org/html/draft-ietf-httpbis-cookie-prefixes-00#section-3.2).

There's no need to set it to the host by default, so make it optional. If it is
set to a non-empty value, still output a warning if it is not a suffix of the
host, as that's likely not wanted.

Fixes #352.